It's not necessarily irrational. For example, for some people if they ever have any extra money, someone else will immediately spend it for them. If the earner wants to make a larger purchase, perhaps something that will cost short term but pay off in the long term, they need some mechanism to save, outside of the regular controls that apply to daily life.

You may think this situation is still irrational, that the other person is being irrational. But again, there are many life situations out there. Perhaps they have lived in situations where they had to fight for what they needed. Perhaps they lived with an earner who would spend their money on drugs if it wasn't taken away, and yet if the non-earner saved it up themselves, the earner would find it and spend it.

The supposedly rational thing may depend on everyone around you to also be rational, and everyone around them, etc. And given that we are human, and humans are not fully rational...